Transport refrigeration units are called upon to control the temperature of a served space defined by a truck, trailer, container, and the like, to a selected set point temperature in a range between about -20 degrees F. and +70 degrees F., depending upon the cargo being transported. The selected set point temperature in this wide range must be achieved, notwithstanding that the ambient temperature may vary between the highest summer and lowest winter temperatures encountered during severe heat and cold waves.
To get more refrigerant into a heating cycle, it is common to pressurize the refrigerant receiver during a heating cycle, forcing refrigerant out of the receiver and into an active refrigerant circuit.
The problem of trapping refrigerant in the receiver, which is worse during low ambients, is even more critical when the transport refrigeration unit is serving a compartmentalized space, which includes two or more compartments to be conditioned. The main refrigeration unit serves the space directly adjacent to where the main unit is mounted, and the main refrigeration unit also serves remote spaces via remote evaporators disposed in such remote spaces. During low ambients weather caps or front shutters, such as the shutter arrangement disclosed in U.S. Pat. No. 4,527,400, are often used in order to increase the heating capacity of the unit. During high ambients, above about 50 degrees F., for example, there may be too much refrigerant in active circulation, especially in compartmentalized systems having one or more remote evaporators causing high compressor discharge or head pressures which may overload the compressor's prime mover. A throttling valve is usually required to control the amount of refrigerant returning to the compressor in order to prevent such overloads. If the compressor head pressure gets too high, a high pressure cut out (HPCO) detects the high pressure and either shuts down the prime mover, or disengages a clutch on systems having a clutch between the prime mover and compressor.